Volume 5 Preprint 13
Erosion-Corrosion of Duplex Stainless Steel in Flowing Seawater Containing Sand Particles
E.A.M. Hussain and M.J. Robinson
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Volume 5 Preprint 13
EROSION-CORROSION OF DUPLEX STAINLESS STEEL IN
FLOWING SEAWATER CONTAINING SAND PARTICLES
E.A.M.Hussain+ & M.J.Robinson
School of Industrial & Manufacturing Science, Cranfield University, Bedford MK43
OAL, UK, firstname.lastname@example.org
Present address; Kuwait College of Technological Studies
SAF2205 duplex stainless steel was tested in a jet impingement apparatus using
flowing artificial seawater containing sand particles. Erosion-corrosion was
measured under a range of hydrodynamic conditions by recording the increase in
anodic current density that occurred when the passive film was damaged by
particle impacts. The current density increase was shown to be linearly related to
the mean kinetic energy of the sand particles. The surface oxide film developed
optical interference colours in flowing seawater when the stainless steel was held
at anodic potentials and these colours were used to identify the film thickness.
The highest rate of erosion-corrosion occurred in the stagnation region,
immediately beneath the jet, where the particles impacted the surface at an angle
of 90°. The results are discussed in terms of the rates of particle impacts and their
effects on the successive processes of oxide film damage and repassivation.
Erosion-corrosion, duplex stainless steel, passive film, interference colour, kinetic
This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It
will be reviewed and, subject to the reviewers’ comments, be published online at http://www.umist.ac.uk/corrosion/jcse in
due course. Until such time as it has been fully published it should not normally be referenced in published work. © UMIST
This paper describes an investigation of erosion-corrosion of SAF2205
duplex stainless steel caused by sand particles entrained in flowing
seawater. Duplex grades of stainless steel generally display good
corrosion resistance in seawater and the extent of this resistance
depends on the range of potential, temperature and chemical conditions
over which the film remains stable. Within this stable range, disruption
of the film by an erosion process results in spontaneous repassivation of
the surface and a small corrosion current is produced. The combination
of these two events is often described as erosion-corrosion. In many
instances, the two processes together result in a greater rate of metal
loss than the sum of the two processes occurring individually and
synergism is evident
In the present study the experimental conditions chosen were sufficient
to damage the passive film without causing substantial mechanical
damage to the substrate. The sand particles had relatively low kinetic
energies such that the rate of pure erosion to the stainless steel was very
small. These conditions were ideal for observing sensitive changes in the
oxide film thickness caused by different hydrodynamic conditions in the
water jet and for measuring the current densities produced when the
films were disrupted by particle impacts.
The tests were performed on duplex stainless steel SAF 2205 (UNS31803)
with the composition shown in Table 1.
Table 1. Composition of Duplex Stainless Steel SAF 2205
Erosion-corrosion experiments were performed on a test specimen
consisting of three cylindrical electrodes machined from SAF 2205 steel
with the dimensions shown in Figure 1. An electrical connection was
attached to each electrode and they were then arranged concentrically
and mounted in epoxy resin. The surfaces were ground and polished
down to 1 µm with diamond paste and then degreased and ultrasonically
cleaned in isopropanol and air dried.
Fig 1. Arrangement of the three cylindrical electrodes
Closed Loop Jet Impingement Apparatus
The electrodes were mounted directly beneath the orifice of a jet
impingement apparatus as illustrated in Figure 2. The loop contained
approximately 3 litres of artificial seawater and was capable of producing
a velocity of 8.5 ms-1 at the 5 mm diameter orifice. A 1.5 litre capacity
glass cell contained the samples, together with a platinum counter
electrode and standard calomel reference electrode. The seawater
temperature was controlled at 24°C using an electrical heater and
thermostat together with a water-cooled heat exchanger. In some of the
experiments sand was introduced to the flow. The sand particles were in
the range 250-300 µm in diameter, varying in shape from rounded to
Fig 2. Schematic diagram of the closed loop jet impingement apparatus
Typical flow characteristics due to jet impingement on a flat plate
shown in Figure 3. These conditions are assumed to apply to the
seawater jet on the surface of the electrodes used in this study. A
stagnation zone formed directly under the orifice (Region A). The flow
then changed from axial to radial and remained laminar as it accelerated
to its maximum velocity at r/ro = 2 (where ro is the radius of the orifice).
At this point a transition occurred from laminar to turbulent flow and
between r/ro = 2 and r/ro = 4 there was a region of high turbulence
(Region B). At larger radial distances more fluid was entrained in the
flow, the thickness of the wall jet increased and its turbulence decayed
rapidly (Region C).
The inner electrode (Ring 1), which was 5 mm in diameter (Figure 1), was
located within the stagnation zone. The second electrode (Ring 2) was 15
mm diameter and positioned in the high turbulence region (r/ro = 3).
The diameter of the third electrode (Ring 3) was 25 mm diameter (r/ro =
5) so that it would be in the low turbulence region.
Fig 3. Hydrodynamic characteristics of jet impingement on a flat plate
(Efird et al
The shear stress on the surface, τw, can be calculated from the following
τw = 0.179 ρ Uo Re-0.182 (r/ro)-2
where ρ is the density of the fluid
Uo is the jet velocity
Re is the Reynolds number at the orifice and;
Re = (2 ro Uo / ν)
r is the radial distance from the centre of the jet
ro is the orifice radius
ν is the kinematic viscosity
Film Thickness Measurement
Background The thickness of the oxide film that developed from the
reaction between the duplex stainless steel and the flowing seawater was
assessed from its optical interference colour.
Interference occurs between light reflected from the surface of the oxide
film and light returning after reflection at the oxide/metal interface
provided that their paths differ by an odd number of half wavelengths
Owing to the high refractive index of the oxide, the light inside the film is
almost normal to the surface for a wide range of angles of incidence.
Hence, it can be shown that
Path difference = 2tµ = (2n – 1) λ /2
where n = 1, 2, 3 etc
t = thickness of the oxide film
λ = the wavelength most strongly absorbed in the incident light
µ = refractive index of the oxide at this wavelength
The maximum interference takes place when t = λ /4µ in the first
absorption band (1st order), when t = 3λ /4µ in the second and when t =
5λ /4µ in the third. The value of n is limited only by the absorption and
scattering of light in the film.
As the oxide film thickens the shorter wavelengths of light at the violet
end of the visible spectrum are absorbed first. When viewed in white
light, the removal of this wavelength gives the film the complementary
yellow colour. Thickening of the film results in absorption of longer
wavelengths, moving progressively towards the red end of the spectrum,
and its appearance changes through a sequence of complementary
colours shown in Table 2. Second and higher order bands of colours
follow in a characteristic sequence, sometimes known as Newton’s series.
The exact colours displayed can vary depending on the metal substrate
on which the film is growing
Table 2 Complementary colours displayed by destructive interference
Phase change When light is reflected at an interface between media of
different optical density a phase change can occur
which is equivalent
to an additional film thickness C. In consequence, the maximum
interference would occur at film thicknesses of λ /4µ -C, 3λ /4µ - C, 5λ
/4µ - C etc. However, for a transparent oxide film on a metal substrate it
is usual to assume that no phase change takes place and that C = 0.
Measurement Technique On removal of the stainless steel sample from
the flow rig the surface was photographed and the interference colours at
a range of radial distances were compared with those in the Michel-Levy
The path difference was identified for each colour, enabling the
film thickness to be calculated for each region using equation . The
values of refractive index were corrected for wavelength using Cauchy’s
in equation  and are shown in Table 2.
µ = 1.35 + 18.8 x 104/ λ
The influence of the seawater hydrodynamics and sand particle erosion
on the passivity of SAF 2205 was measured by performing
potentiodynamic scans on each electrode. The potential was scanned
from -600 to 1300 mV(SCE) at a rate of 10 mV/min using a Sycopel
Scientific computer controlled Autostat 251.
The resulting polarisation behaviour was used to identify the potential
range over which the passive film was stable. A single potential was then
selected at which localised corrosion of one or both of the
microstructural phases was expected to occur, and cylindrical samples
approximately 30 mm in diameter were tested in flowing seawater at this
potential for times up to 30 hours.
The general characteristics of the polarisation scans for SAF 2205 in
flowing seawater are shown in Figure 4. These results were obtained on
the outermost electrode (Ring 3), located in the low turbulence region. In
this position the polarisation behaviour was affected little by flow rate
and even the addition of sand particles did not affect the curve, apart
from creating small current fluctuations in the passive range. These
fluctuations were the result of depassivation and repassivation events on
the steel surface associated with the particle impacts.
An important feature of the polarisation behaviour of the duplex stainless
steel was the existence of two discrete pitting potentials. The steel was
passive at the open circuit potential and remained so up to 400 mV(SCE).
Above this potential pitting occurred in the ferrite phase, while the
austenite remained passive, until a potential close to 900 mV(SCE) was
reached where pitting occurred in both phases. Metallographic
examination confirmed that in the range 400-900 mV(SCE) pitting was in
the ferrite alone
Fig 4. Potentiodynamic polarisation scans for SAF 2205 in low turbulence
flowing seawater on Ring 3
The largest influence of flow and sand erosion on the polarisation
behaviour was recorded on the central electrode (Ring 1), positioned in
the stagnation region directly beneath the orifice. Figure 5 shows the
results for a velocity of 8.5 ms-1 both in flowing seawater alone and with
the addition of 1, 2 and 3 gm of sand. Each of the polarisation scans had
the same general features as in the low turbulence region on Ring 3,
shown in Figure 4, and exhibited a passive range and separate pitting
potentials for the ferrite and austenite phases.
Without sand additions, the current density in the passive range was
essentially the same on Rings 1,2 and 3. However, the addition of sand
particles caused a systematic increase in the current densities recorded
on ring 1, beneath the orifice. This was most pronounced in the passive
range, where an increase of an order of magnitude occurred, but a
smaller increase also took place in the ferrite pitting range. The increase
in passive current density with the addition of sand also displaced the
intersection of the anodic and cathodic polarisation curves and resulted
in a lowering of the open circuit potential as illustrated in Figure 6.
Fig 5. Potentiostatic polarisation scans for SAF 2205 in the stagnation
Fig 6. diagram to illustrate the effect of flow rate on the open circuit
potential of SAF 2205 in flowing seawater containing sand particles
Effect of Applied Potential Pronounced interference colours were visible
on the electrode surfaces after exposure to flowing seawater for six or
more hours at the anodic applied potentials. Similar effects have been
reported in other studies
The colours were used to study the effect
of the different hydrodynamic conditions beneath the jet and the erosion
caused by the sand particles.
Figure 7 shows samples that had been exposed to jet impingement at a
velocity of 8.5 ms-1 and held at 400, 700 and 900 mV(SCE) to correspond
to the ferrite pitting potential, the ferrite pitting/austenite passive range
and the austenite pitting potential, respectively. The colours differed at
each potential but all displayed essentially three zones which
corresponded to the stagnation, high turbulence and low turbulence
Fig 7. Film thickness measurements for SAF 2205 exposed to seawater at
8.5 ms-1 with potentials of 400, 700 & 900 mV(SCE)
The sequence of interference colours identified them as being in the
second and third orders. The film thickness was then analysed from its
colour using equation  and the refractive indices given in Table 2. The
results are shown graphically in Figure 7. The thickness was similar in
the stagnation and low turbulence regions but considerably thinner in the
high turbulence region. This is thought to be due to the high surface
shear stress in that position.
A similar trend was found at a jet velocity of 7.9 ms-1, as shown in Figure
Fig 8. Film thickness measurements for SAF 2205 exposed to seawater at
7.9 ms-1 with potentials of 400, 700 & 900 mV(SCE)
Effect of Sand Erosion An oxide film was first developed on the electrode
surface in flowing seawater by holding it at a potential of either 400, 700
or 900 mV(SCE) for six hours. The specimen was removed from the flow
loop and the film photographed and its thickness analysed, as described
above. It was then returned to the loop and tested at the same potential
in flowing seawater with the addition of 3 gm of sand.
The effect of adding sand was to erode the oxide film, particularly in the
stagnation region beneath the orifice. The interference colours
remaining after exposure at velocities of 8.5 and 7.9 ms-1 are shown in
Figures 9 and 10, respectively. The original film was thin in the high
turbulence region and the sand particles appeared to have little
additional effect due the low angle of the impacts on the surface.
Similarly, no film thinning was recorded in the low turbulence region.
Short exposure times of 10-60 minutes removed the oxide film in the
stagnation region alone (Figure 10), whereas longer exposure (6 – 18 hr)
progressively removed the oxide over wider areas of the surface (Figure
In the stagnation region the film was a first order grey-blue colour for all
exposure times. This film colour corresponded to a thickness of
approximately 33 µm, which suggests that the film was immediately
reforming after being removed by the sand erosion.
Fig 9. Film thickness measurements for SAF 2205 in seawater containing
3 gm sand at 8.5 ms-1 with potentials of 400, 700 & 900 mV(SCE).
Fig 10. Film thickness measurements for SAF 2205 in seawater
containing 3 gm sand at 7.9 ms-1 with potentials of 400, 700 & 900
mV(SCE) for 1 hour.
Samples of water/sand were collected from the loop in order to measure
the quantity of sand that was suspended in the flow and to allow for any
that had settled out. From these measurements the rates of particle
impacts were calculated. For the addition of 3 gm of sand, the rates were
2400 and 1400 impacts/second on Ring 1 at velocities of 8.5 and 7.9 ms1,
respectively. It follows that the corresponding average times between
particle impacts over the electrode surface were 0.42 and 0.71 ms.
Modelling Kinetic Energy of Particle Impacts It was assumed that the
increase in passive current density measured during erosion-corrosion of
the duplex stainless steel in seawater containing sand particles was due
solely to disruption and repair of the oxide film. Furthermore, it was
expected that the current density would be directly related to the total
kinetic energy of the impacting sand particles.
The total kinetic energy of particles impacting the surface in one second,
KE, is given by;
KE = ( N mav V2 ) / 2
where N is the number of impacts per second
mav is the average mass of a sand particle
V is the flow velocity
Figure 11 shows the passive current density in the stagnation region
(Ring 1) plotted against the total kinetic energy per second of the
impacting particles for two different flow velocities. The straight line
graphs confirm the assumed relationship between the passive current
and the impact
Passive Current Density (microamps/cm2)
Kinetic Energy of Impacting Particles (Jsec-1)
Fig 11 Graphs of passive current densities for SAF2205 in seawater at 0
mV(SCE) plotted against total kinetic energy per second of impacting
Similar results were obtained in an earlier study on Type 304 austenitic
as shown in Figure 12.
Passive Current Density (microamps cm-2)
Total Kinetic Energy of Impacting Particles (J sec-1)
Number of Impacts
Number of Particle Impacts (sec-1)
Fig 12 Effect of the number and total kinetic energy of particle impacts
on passive current density for Type 304 stainless steel in flowing
In order to relate the passive current densities to the film thickness
measurements it was necessary to consider the time interval between
particle impacts at each point on the surface.
The number of impacts per second, n, on a single point
n = N aimp/As
is given by;
aimp is the mean impact area
As is the area of the stagnation region (Ring 1)
For 3 gm sand and an orifice velocity of 8.5 ms-1, the passive current
density at 0 mV(SCE) was 7.5 µAcm-2 (Figure 5). The mean impact crater
was measured by scanning electron microscopy to be 11.8 µm in
diameter, giving an impact area of 1.1x10-6 cm2. It follows that for N =
2400, as given above, the mean number of impacts on each point of the
surface, n, was 0.013 per second. Therefore, the mean interval between
these impacts would have been in the order of 75 seconds. This time is
thought to be more than sufficient for the film to reform at the point of
impact and for some thickening to occur
The fact that a discernable colour was present directly beneath the orifice
(Figures 9 & 10) indicates that the film was reforming rapidly between
particle impacts and that film removal and repair were occurring
simultaneously, as competing processes. In flowing seawater, without
sand additions, the passive current density was close to 0.8 µAcm-2
(Figure 5) and the additional current measured when sand was added
represented that required to repair the film. However, the typical bluegrey colour of the film (33 µm thickness) shows that there was
insufficient time between particle impacts for the film to grow to the
same steady state thickness that developed in flowing seawater alone.
As a point of interest, the charge passed for each particle impact may be
estimated from the data given in the example above. A current density of
7.5 µAcm-2 measured in the stagnation region, represents an increase in
total current of 1.3 µA over the area of Ring 1. Clearly, in one second a
total charge of 1.3 µC would pass in reforming the passive film, as a
result of 2400 impacts. Therefore, the mean charge passed per impact
may be estimated as 5.4 x 10-10 C.
The charge passed when a passive film on iron or stainless steel is
mechanically damaged has been investigated by other researchers
In each case, they recorded a large initial current, which gradually
decayed. In the present study, the number of impacts per second, n, on
each point of the surface was relatively small, such that the passivation
current decayed substantially before a further impact occurred at the
same position. Two extreme cases are represented schematically in
Figure 13. In case (A), where n is small, the passivation current decays
completely before the film is again disrupted by another impact. In
contrast, case (B) for large values of n shows overlapping decay transients
resulting from multiple impacts. In principle, with a sufficiently high rate
of impacts a limiting situation could be reached in which the surface
would remain active, such that additional impacts would not cause any
further increase in the current density. Clearly, the results in the present
investigation are closer to case (A).
Fig 13. Schematic diagrams representing passivation and depassivation
events resulting from particle impacts
 The erosion-corrosion of SAF2205 duplex stainless by sand particles
suspended in flowing seawater was shown to be controlled by both the
hydrodynamic conditions and the particle kinetic energies.
 The highest erosion-corrosion rate occurred in the stagnant region,
immediately beneath the jet, where sand particles impacted the surface at
an angle of 90°. At greater radial distances the flow became highly
turbulent but the erosion-corrosion rate was reduced due to the lower
angle of particle impacts.
 Impact damage to the oxide film resulted in an increase in the anodic
current density due to repassivation of the surface. A linear relationship
existed between current density and the mean kinetic energy of the sand
 The oxide film that formed in flowing seawater at applied anodic
potentials thickened to produce optical interference colours. The colours
were a useful measure of the effects of the hydrodynamic conditions and
the damage caused by sand erosion.
 At low rates of particle impacts the surface repassivated between
successive impacts and produced a visible interference colour but there
was insufficient time for significant film thickening to occur.
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